$files = (New-Item -Type File -Force '.\About_Arrays\temp\t1.txt'),
(New-Item -Force -Type File '.\About_Arrays\temp\t2.txt')

$files.LastWriteTime

# $files.LastWriteTime = (Get-Date).AddDays(-1)

$files.set_LastWriteTime((Get-Date).AddDays(-1))
$files.LastWriteTime

$files | Get-Member | Where-Object Definition -like '*set;*'

